Three induced mutations of the mouse are true alpha-thalassemias which map to or near the Hba alpha globin structural gene locus. Although two were independently induced with x-rays and the third with triethylenemelamine, all appear to have a similar cause: an incomplete deletion of the alpha gene complex. In each, the adult globin genes were previously shown to be fully inactive: we now have proven that at least one (probably both) of the two adult alpha genes per chromosome is actually deleted. Other parts of the alpha locus complex are still present but these apparently are not being expressed as proteins at detectable levels. Apparently normal variations at the globin gene DNA level have also been found among inbred and exotic mouse stocks. Messenger RNA levels in the thalassemia heterozygotes are around 80% of normal, implying effective compensation for the loss of one gene set.